1. Field of the Invention
The present invention relates to a dynamic mode AFM apparatus.
2. Description of the Related Art
Firstly, an AFM (atomic force microscopy) will be described.
A contact mode AFM is a technique to image a “constant force surface” of a sample surface by detecting force (usually, repulsive force), which is exerted between a probe and a sample when a cantilever with the probe attached thereto is brought close to the sample surface, based on flexure of the cantilever, and by two-dimensionally scanning the sample with the probe while controlling a probe-sample distance so that the detected force is kept constant. This contact mode AFM gives substantial damage to the sample due to the strong force exerted between the probe and the sample, and the atomic resolution is difficult to achieve.
In contrast, a dynamic mode AFM is a technique to image a “constant force gradient surface” of the sample surface by bringing a cantilever with a probe attached thereto close to a sample surface, detecting change in a resonance frequency of the cantilever due to a differential (force gradient) of force exerted between the probe and the sample with respect to a probe-sample distance, and two-dimensionally scanning the sample with the probe while controlling the probe-sample distance so that the change in the resonance frequency is kept constant.
FIG. 1 shows an exemplary configuration in the area of a sample and cantilever of a conventional dynamic mode AFM apparatus.
In FIG. 1, reference numeral 201 denotes a sample, 202A denotes a probe of a cantilever 202, 202B denotes a base of the cantilever 202, 203 denotes an XYZ scanner, 204 denotes a cantilever excitation means, 205 denotes an optical position detector (detector with an optical lever) to detect the position of the cantilever 202 by irradiating a bottom face of the cantilever 202 with a laser beam 206, and 207 denotes a state of flexural vibration of the cantilever.
FIG. 1 shows X, Y, and Z directions because the XYZ coordinate will be used in the following description. Although the sample 201 is mounted on the XYZ scanner 203 in this example, there are other variations in which the cantilever 202 is attached to the XYZ scanner 203, or the sample 201 is attached to an XY scanner and the cantilever 202 is attached to a Z scanner. Moreover, although the figure illustrates the cantilever excitation means 204 similar to a piezoelectric element, it is also possible to utilize photothermal excitation or electromagnetic field. Furthermore, although the optical position detector 205 is used to detect the flexure of the cantilever 202 with the optical lever, it is also possible to apply speed detection by a laser Doppler vibrometer or displacement detection by an optical fiber interferometer.
FIG. 2 shows an exemplary relationship between the probe-sample distance and a force and force gradient acting on the cantilever, and FIG. 3 shows an exemplary relationship between the probe-sample distance and the resonance frequency of the cantilever. The reason why the resonance frequency of the cantilever varies due to the force gradient is that the force which varies dependent on the distance is equivalent to a spring and thus the force acted by the equivalent spring is added to that of a spring inherently provided for the cantilever. However, the equivalent spring will have a negative spring constant when the polarity of the force gradient is positive. When the negative spring constant is applied, the resonance frequency will decrease.
Methods to detect the change in the resonance frequency include: (1) a method in which the cantilever itself is used as a mechanical resonator to configure a self-excited oscillation circuit to detect the change in the oscillating frequency; and (2) a method in which the cantilever is forced to vibrate at a constant frequency near the resonance frequency to detect the change in the resonance frequency from a phase difference between a signal used for the vibration and the detected vibration. Assuming that the above methods (1) and (2) are referred to as the FM (frequency modulation) method and the PM (phase modulation) method, respectively, there is a third method (3) in which, while the forced vibration is used, the frequency for the forced vibration is controlled to follow the resonance frequency by utilizing the detected phase difference. Here, this method is referred to as the tracking separate-excited method.
Since any method above can detect information on a frequency axis with high sensitivity by narrowing a bandwidth to be observed, the dynamic mode AFM allows observation in a region where the probe-sample force is weak as compared to the contact mode AFM, resulting in less damage to the sample and thus the atomic resolution can be obtained more easily.
As described above, the dynamic mode AFM traces the “constant force gradient surface”. The “constant force gradient surface” is generally considered to approximate a “constant height surface”. Since the force gradient graph of FIG. 2 varies dependent on atomic species, however, the “constant force gradient surface” would be identical to the true “constant height surface” only in the case where the force gradient graph of FIG. 2 does not change while the sample consists of single-element atoms and the probe tip is placed right above an atom or between atoms. Therefore, for the sample consisting of atoms of plural elements, the “constant force gradient surface” is not identical to the true “constant height surface”, and the observed atomic species cannot be estimated unless some information on constituent elements or crystal structures of the sample has been preliminarily provided.
Meanwhile, the literature has been published that describes the position of the minimum point (point B where the resonance frequency decreases most, i.e., the point where the force gradient of FIG. 2 is maximum) in the graph of FIG. 3 is characteristic of the atomic species, and thus the atomic species can be determined by obtaining the minimum point position (see Non-Patent Document 1 below).
According to this method, it is possible to color a topographic image (three-dimensional graphic representation of the “constant force gradient surface”) of the sample observed by the conventional dynamic mode AFM based on the atomic species obtained from the minimum point position, so as to display the image as if each atomic species is differently colored.
Non-Patent Document 1: Yoshiaki Sugimoto et al., “Chemical identification of individual surface atoms by atomic force microscopy”, Nature, Vol. 446, 2007, pp. 64-67